As methods of protecting the surfaces of a wide variety of materials, there have so far been known a method of bonding a protective sheet having an adhesive layer on one side of a thin film, such as a polyester film, to the surface of a subject to protect and a method of forming a protective layer on a film having releasability and transferring the protective layer to the surface of a subject to protect through the medium of a pressure sensitive adhesive or a general adhesive. The method of transferring a protective layer to the subject surface via a pressure sensitive adhesive or a general adhesive has an advantage that a thin protective layer can be formed, but it has also a disadvantage of failing to ensure sufficient strength in the transferred layer as a whole because the strength of the pressure sensitive adhesive or general adhesive layer is lower than that of the protective layer. In order to overcome this disadvantage, various studies have been hitherto made.
For instance, Japanese Patent Laid-Open Nos. Sho 64-18698 and Hei 4-201478 disclose the inventions in which the protective layer and the adhesive layer made up of an ionized radiation-curable resin which is a solid in an uncured state but has thermal plasticity is provided on the releasable surface of a release sheet. In addition, Japanese Patent Laid-Open No. Hei 7-125496 discloses the transfer sheet having on a base film having releasability a subbing layer and a non-adhesive semi-cured layer in a gelled state formed by irradiating a UV-curable resin coating with UV rays in an exposure dose about ⅓ to about ⅔ the exposure dose required for completely curing the resin coating.
However, since the layer brought into contact with a subject of protection has a non-adhesive surface, those transfer sheets have a defect that heating treatment at the time of bonding or additional application of a UV-curable resin to the subject surface is required for bonding the transfer layer to the subject surface and thereby the protective layer formation process becomes complicated. Further, such transfer layers have a drawback of being unfit for transfer to materials sensitive to heat or solvents (e.g., emulsion surfaces of photographic films utilized as photomasks).
In addition, Japanese Patent Laid-Open No. Sho 61-258742 proposes a method of transferring a protective layer having both releasability from a support and adhesion to the support to the surface of a subject to protect through no medium or the medium of a combination of hot-sealable and pressure sensitive adhesive layers. In this method also, heating is required at time of transfer. The use of a pressure sensitive adhesive layer with the intention of avoiding the necessity for heating caused a defect that the protected layer formed was poor in surface strength.
On the other hand, a photomask is generally made up of a transparent substrate and an emulsion layer provided thereon, and it is a sheet wherein patterns formed by a CAD system or the like are reproduced by subjecting the sheet sequentially to an exposure with a device like a photoplotter, development such as washing, fixation and drying. The photomask in which the desired patterns are formed is often used as an original in the so-called photoetching process. More specifically, a photopolymer-containing layer is exposed via the photomask in a state that the photomask is kept in close contact with the layer or superimposed on the layer with a slight gap between them, then developed, and further dried. Thus, patterns corresponding to the patterns in the photomask are formed in the photopolymer-containing layer.
Incidentally, the emulsion layer used for a photomask contains gelatin as a main component, and the pencil hardness thereof is 2B or below. Therefore, when the contact exposure in a photoetching process is carried out in a condition that the emulsion layer surface is brought into close contact with the photopolymer layer as a subject without providing any special protective layer, the photomask tends to receive flaws on the surface to result in a partial loss of its pattern data. Further, the emulsion layer surface is contaminated with microbes to cause a loss of information on the photomask or defective shooting.
Under these circumstances, the aforementioned defects have so far been compensated for by bonding a commercial pressure sensitive adhesive-applied film to the emulsion surface of an unprotected photomask (the term “an unprotected photomask” as used herein means a photomask which is provided with no protective layer or the like but has an emulsion layer surface in a bare state). However, such a pressure sensitive adhesive-applied film has a drawback of being sensitive to scratches of hard foreign materials because the layer of the pressure sensitive adhesive is soft and the film itself tends to suffer scratches. Therefore, increase in the number of times such a photomask has been used necessitates replacing the protective layer of the photomask by a new one. Therefore, it cannot be said that commercial pressure sensitive adhesive-applied films function satisfactorily as protective layers.
When the pressure sensitive adhesive-applied films are increased in film thickness for diminishing the foregoing drawback, the protective layer strength is heightened, but the UV-ray transmittance as a photomask's optical characteristic is lowered. As a result, it becomes necessary to extend the exposure time, and what is worse, high-definition patterning becomes impossible because reflection, refraction and scattering of light reveal their influences. In other words, in the case of using pressure sensitive adhesive-applied films, the photomask's drawback of being sensitive to flaws cannot be overcome by adjustment of film thickness since the film thickness decreased to an extent that no problems are caused in the foregoing aspect of optical characteristics cannot secure the strength as the protective layer (the order of B in pencil hardness). In addition, such thin films tend to become wrinkled at the time when they are bonded to unprotected photomasks, so they cause lowering of workability. Accordingly, the pressure sensitive adhesive-applied films can hardly satisfy all the requirements, sufficient protective layer strength, satisfactory UV-ray transmittance and good workability.
Furthermore, as described in Japanese Patent Laid-Open No. Hei 11-305420, it has been tried to provide protective layers by applying coating materials on unprotected photomasks according to a variety of methods, such as spray coating, spin coating and dip coating methods, and then thermally curing them simultaneously with drying. However, such methods require a heat-applied aging time of 18 to 72 hours, so there is the fear of a damage to the emulsion surfaces. In addition, it is hard to control the coating layer thickness in the foregoing coating processes (wet processes), so it has been difficult to put such methods to practical use.
Therefore, a first object of the present invention is to provide a transfer sheet which enables easy formation of a protective layer having good contact with an emulsion layer and high resistance to scratching, even when the emulsion surface is poor in heat resistance and solvent resistance.
A second object of the present invention is to provide a protective layer-attached photomask having excellent optical characteristics and high resistance to scratching.